This invention relates to ceramic metal halide (CMH) lamps and the sealing technology of such lamps.
Often times one of the structural components in the electrical feedthroughs of such lamps is made of a cermet (ceramic-metal composite) material. Cermets have been known for a long time to provide acceptable solutions for the sealing of electrical feedthroughs to surrounding nonconductive materials. For example, cermet materials have been made as early as 1979 by mixing coarse refractory oxide granules with fine metallic powders, such as tungsten, nickel and molybdenum, to obtain suitable electrical conductivity therein and yet result in having thermal expansion coefficients compatible with ceramic materials. Typical ceramic materials used in CMH lamps are polycrystalline alumina (PCA), individual rare earth oxides or their mixtures and sapphire.
In later years, up to the early 1990's, details of making cermets with various particle size materials, their structural forms, and their initial use in ceramic metal halide lamps were described by various lamp developers, but they did not at that time result in a practical ceramic metal halide (CMH) lamp. Later, in the mid 1990's, the first commercially viable CMH lamp was introduced, and the performance of metal halide lamps got a big boost as a result since the color characteristics, the kind of chemistries used, and the efficacies obtained were far superior to the previous quartz metal halide lamp technology. While the initial lamps introduced had electrical feedthroughs with structures made of niobium (Nb), molybdenum (Mo) and tungsten (W) metals, at later times some CMH lamps were introduced using cermets in those structures. Much of the work attempted to either shorten the overall size of an extended plug structure, lower the cost of the materials used, increase reliability of the seal under high temperature conditions, or provide an alternative seal that would be more manufacturable, or some combination of these. Here, the term “feedthrough” will be used for the entire current carrying structure of metals and ceramics, and the term “electrode” will be used specifically for the very tip section of the electrical feedthrough that is in contact with the gas plasma and the discharge arc during lamp use.
As is well known, the extended plug structure of such lamps, as shown in FIG. 1, allows the temperature at the seal reached during lamp operation to be considerably lower than that reached using a non-extended plug structure. A lamp, 4, in that figure has a borosilicate glass envelope, 5, set in a conventional Edison-type metal base with lead wires 6 and 7 leading therefrom supported in the envelope by a glass flare 8 and a glass dimple 9. Lead wires 6 and 7 are connected to the leads, or electrical feedthroughs, of an arc discharge tube 10. The reason for this temperature improvement is that the extended plug construction positions the seal further from the electrode in the tube main chamber in which the arc discharge occurs during lamp operation, and where temperatures are greatest in the electrical feedthrough during such operation, as compared to the non-extended plug structure that has the seal relatively close to the electrode in the main chamber (essentially without a PCA capillary extension). This feature of relative remoteness enables the extended plug structure types of lamps to have a reasonable duration of lamp operations, or lamp lifetime, and so be commercially viable.
One of the electrical feedthrough structural arrangements available involves the use of cermets that have an expansion coefficient intermediary to those of the two materials used in the structures joined therewith (which materials are also used in formulating the cermet material)—most often polycrystalline alumina and molybdenum. Although the cermet material allows forming a successful hermetic seal between the electrical feedthrough, in which it is used as a structural component, and the PCA of the capillary tube thereabout in which it is positioned via the glass frit sealing material, the cermet material also tends to be fairly brittle and a difficult base on which to spot weld other structural components to it. Therefore, it is quite a task in manufacturing arc discharge tubes and the corresponding lamps to handle the ends of electrical feedthroughs as arc tube electrical leads with a cermet piece as part of them sticking out of the PCA capillary to be exposed to the various risks and stresses of the manufacturing process.
The extended plug structure arc discharge lamp arrangement in present use that avoids having any cermet portion in the electrical feedthrough being exposed to the exterior of the arc discharge tube is shown in the cross section view in FIG. 1A of arc discharge tube 10 from FIG. 1. This is an arc discharge tube for a 150 W ceramic metal halide lamp 5 and, as indicated, is formed with an extended plug structure. Here arc discharge tube 10 includes a cylindrical main tube portion 15 smoothly joined with tapered capillaries portions 11a and 11b. Main tube portion 15, as well as the capillary parts 11a and 11b of the arc tube, are typically made of translucent ceramic material in which alumina is a main component.
A partially externally exposed outer lead wire, or sealing member 16a, a first lead-through wire 19a, and a first main electrode shaft 21a are joined together to form an electrical feedthrough that is positioned in capillary part 11a. In this arrangement, lead-through wire 19a is of a cermet material and sealing member 16a is a niobium material rod. A material typically used for electrode shaft 21a is tungsten or molybdenum. Specifically, one end of lead-through wire 19a is connected with one end of sealing member 16a by welding, and the other end of lead-through wire 19a is connected with one end of main electrode shaft 21a again by welding. Sealing member 16a is fixed to the inner surface of capillary part 11a by a glass frit 17a such that sealing member 16a is sealed hermetically to capillary part 11a. Sealing member, or outer lead wire, 16a is typically formed by niobium wire of a diameter compatible with the expansion coefficient of frit 17a. For example, the diameter of a niobium outer lead wire 16a may be 0.9 mm and the diameter of a molybdenum first main electrode shaft may be 0.5 mm.
Sealing member or outer lead wire 16a, first lead-through wire 19a and first main electrode shaft 21a are disposed, as indicated, in the capillary part 11a such that an end portion of outer lead wire 16a is positioned outside capillary part 11a. As can be seen in the figure, however, niobium outer lead wire 16a is positioned with the inner end thereof deep into capillary part 11a so that only a small thickness of frit 17a covers that inner end thereof. Thus, the repeated heating and cooling of arc tube 10 between the lamp being operated and not operated at some point tends to lead to cracks in frit 17a. Such cracks need not go far before outer lead wire 16a is exposed to the salts developed in the main discharge chamber during lamp operation which are very destructive of niobium material. Thus, this structural arrangement has a substantial risk of failure.
An electrode coil 22a is joined to the tip portion of main electrode shaft 21a by welding, so that main electrode 23a includes main electrode shaft 21a and electrode coil 22a. The electrode coil 22a is made out of tungsten or doped tungsten. The lead-through wire 19a serves as a lead-through to assure the placement of main electrode 23a at a predetermined position in main tube 15.
An alternative to the FIG. 1A electrical feedthrough structure is shown in FIG. 2 where a cross section view of one side of a PCA capillary part is shown. Here, instead of attaching cermet lead-through 19a directly to the tungsten main shaft 21as in FIG. 1A, a molybdenum mandrel 12 with a fine molybdenum coil 13 surrounding it is inserted between a cermet lead-through 25 and tungsten main shaft 21a. The advantage of this structural arrangement is that the salts formed in the discharge chamber during lamp operation do not penetrate as far into the end of the capillary, and therefore they are not as cold to thereby yield a reasonable performance of the lamp. In addition, the combination of the molybdenum mandrel and coil is compatible with the expansion/contraction of the capillary PCA so as not to lead to cracking thereof.
Also shown in FIG. 2 is the use of a stopping cross wire 29a that is welded to an outer wire 29 to limit the insertion distance of the electrical feedthrough through the capillary part into the arc discharge tube to assure that the feedthrough does not get outer lead wire 16a too near the main discharge chamber and that electrode 22 is properly spaced with respect to its counterpart feedthrough positioned on the opposite side of the main discharge chamber. This is a costly added step to accomplish in the difficult welding situation presented due to the previous nearby weld of outer wire 29 to cermet lead-through 25.
Another alternative (not shown) is to use a crimping operation to widen the cross section of the outer lead wire along a laterally directed cross axis thereof which is provided right at the junction of the cermet lead-through and the outer lead wire weld. Such a widened cross section crimp preventing further insertion of the electrical feedthrough into the capillary part results in accomplishing a similar result in setting the feedthrough insertion depth into the arc discharge tube through a capillary thereof as does the provision instead of a cross wire as describe above. However, the crimping process often leads to geometric distortion of the remaining wire such as bending or twisting it into misalignment with the feedthroughs axis of the lamp. This leads to a relatively large failure rate in the remaining lamp manufacturing process steps which significantly increases manufacturing costs. Therefore, an accurate crimp at the joint of these two materials is a difficult to impractical approach for low cost manufacturing of CMH lamps.
As indicated above, cermets do provide a good solution for structural use as part of an electrically conductive feedthrough in having a thermal expansion that is compatible with surrounding structures in CMH lamps; however, as also indicated above, the brittleness of the cermets and the difficulty of spot welding to them makes using cermet structure portions a difficult choice in manufacturing such feedthroughs. Thus, there is a desire for an electrical feedthrough structure better suited to the use of cermet portions therein.